safer inannex checking

git-annex-shell inannex now returns always 0, 1, or 100 (the last when
it's unclear if content is currently in the index due to it currently being
moved or dropped).

(Actual locking code still not yet written.)

Remote/Web.hs

@@ -64,7 +64,7 @@ dropKey _ = do
 	warning "removal from web not supported"
 	return False
 
-checkKey :: Key -> Annex (Either IOException Bool)
+checkKey :: Key -> Annex (Either String Bool)
 checkKey key = do
 	us <- getUrls key
 	if null us